Abstract: A five-level PE-model was applied to simulate the formation of sub-synoptic systems using real data of rainfall amount and geopotentials. Experiment indicates that due to release of latent heat through condensation process in the numerical model a convergence zone and a sub-synoptic vortex develops in the precipitation region. The low level jet stream with scale about 500 km is located along the south side of heavy rain area. The wind profile, vertical circulation and location of low level jet reletive to the heavy rain area are in agreement with average structures of Mei-Yu front. The dry numerical model can not give similary results.